Emission indicator



Sept. 26, 1950 z, w, wu c ms 2,523487 EMISSION INDICATOR Filed Dec. 10, 1945 vwwwbcw ZIGMO ND W. WILCHINSKY Patented Sept. 26, 1950 UNITED STATES PATENT OFFICE EMISSION INDICA'IOR Zigmond W. Wilchinsky, Washington, D. (3., as-

signor, by mesne assignments, to the United States of America as represented by the Seeretary of the Navy Application December 1=0, 1945, Serial N0. 634,106

4 Claih1s.

electron emitting material are often drawn avvayf1om the cath ode and deposited on the grid. This process, usually called grid Contamination, eventually results in a condition wherein the grid itself emits electrons in substantial numbers when it reaches the relatively high temperatufe at which the grid of a power tube normaplly operates. Substantial thermionio emission from the grid causes erratic tube behavior, which may croseconds, while the inactive interval intervening between successive pu1ses might be of the order of several hundred microseconds. This last-named interval is much shorter than the time required ior a grid to cool appreciably; hence this invention will provide conveniently a continuous check 011 the presence of thermionic grid emission in such anode-pulsed vacuum tubes.

The invention will be described with reference to the accompanying drawing, which is a schematio diagr'am showing an embodiment of the invention incorporated in an anode-pulsed vacuum tube circuit which for illustrative purposes only is shown as a Hartley oscillator.

terest of dependable operation, it is highly denation, in oraler that a contaminated tube may be replaced before rather than after it causes system failure.

It is accordingly an object of this invention to provide a method and means fordetecting the presence and relative magnitude of thermionic grid emission in vacuum tubes.

The operation of the invention rests upon the fact that the grid of a vacuum .tube has appreciable masg and therefore does not cool instantly when the vacuum tube is cut 01T from its power source. Hence it is possible, by suddenly rexnoving the power from the tube, to detect grid emission before the grid cools by means to be hereinafter described.

The method and means of this invention are thus particularly adapted for use with tubes which are in anode-pulsed service, wherein a very large voltage is applied between anode and cathode recurrently for very short periods, separated by much longer periods wherein the anode voltage is zero. This results in operating the tube for short periods at a very high power level separated by much longer periods of inactivity. The type of operation just described is encoun tered in eoho-ranging and pulse-communication systems, and represents a large fraction of modern high-frequexicy applications of power vacuum tubes'. In such applications the period of each pulse of anode voltage might be a, few mi- The vacuum tube to be checked for grid emission is denoted by reference character H], and shown with its cathode grounded. Terminal 30, to Which the negative side of the anode voltage supply is to be connected, is also grounded. The anbde of tube I0 is connected through radiofrequency choke coil 9 to terminal 2ll, to which positive anode voltage impulses are to be applied. The anode of tt1be I0 is also connected through blocking condenser II to one side of a tank circuit consisting of inductance 4 and sondenser 5 in parallel. The other side of the tank circuit is connected through condenser 1 to the positive side of D.-C. source 3. The negative side of D.-C. source 3 is connected to the grid of tube I0. Condenser 1 is shunted by grid leak resistor 6. D.C. source 3 is shunted by R. F. bypass condenser 8. Inductance 4 is grounded via an intermediate tap. D.C meter l is connected between the anode of tube I!) and the cathode of diode 2. The anode of diode 2 is counected to the positive side of D.-C source 3 The circuit elements about which the invention is mainly concerned and shown in the illustrated embodiment, are D.-C. meter I, diode 2, and D.-C. source 3. D.C. meter l may be any sensitive direct-current indicating device, With 01 without calibration in actual units. D.-C. s0urce 3 may be a small battery or any other D.C. source supplying a few volts.

The operation of the invention, as heretofore stated, depends upon the fact that the grid temperature does not drop sharply after the termination of a pulse of anode voltage. Between anode pulses the anode voltage is substantially. zero. Therefore if the griol is emitting electrons at its operating temperature grid-to-anode space-current conduction Will permit a direct current to flovv from D.-C source 3 through diode 2, meter l, and within tube I0 between the anode and grid. If such a current flows, an indication on meter l Will result, serving as a warning that tube IG has a contaminated grid If the grio'l 15 not emitting electrons at its operating temperature, meter then Will at all times read zero.

'Ihe presence of the aforementioned components of the 'invention in'-this circuit arrangement has no adverse efiect on the normal operation of the circuit, since the application to tube II] of Voltage from its power supply causes the cathode of diode 2 to become much more positive .thanits anode. Conseqently, diode 2 is effectiveily an open circuit during the operative intervals of tube I and the presence 0f the inventioniuas n0 effect except the negligible one of slightly'increasing the grid-anode capacitance o1" tube I0.

In practice, if complete absence of -thermionic grid emission is deemed essential, replacement of tube I0 may be eifected immecliately whenvermeter l shows any readrin during system operation. If 1a smallamountof grid:emission is tolera'ble, the critica1-ordanger point may i=be detrmined by -noting the reading of -meter I at Which the contaminated:grid begins to causetrouble. When this point 'has been determined a meter reading somewhat e10w =that .critical va1ue may be selected *as -the -warning signal for tube replacement.

The invention once installed in an anode pulsed vacuum tube circuit, gives a oonstant check 0I1 grid contamination and involves -n0 maintenance problems. If a, batter vis -used as D;-C. source 3, itswlifeis substamtially equal -to shelf 1ife, since 110 current is drawn fromitexoept when grid contamination is present, :and even thenthe drainig -very sma1l.

lt will be understood that the embodiment.of the invention herein shown ancldescribed is exemplary on1y and that the scopeof the invention is to be determined.-by reference to the appended elaims.

What is claimed is:

1. The method of detecting thermionic grid emission in a vacuum taube including at least an anode, a cathode and a grid, comprising suddenly interrupting the operation of the vacuum taube by cutting off its anode-cathode voltage, applying a voltage between grid and anode, and detecting the presence of grid-anodespace current before the grid has appreciably cooled fmm its operating temperature.

' '2. The method of detecting thermionic grid emissionin a recurrently-operative anode-pulsed vacuum 'tube including at least an anode, a .cathoiieand argrid, eomprising applying a Volt- 'age between grid and anode during the interva1s hetween prilses and detecting the presence of g1id-anode space current during such intervals.

3. In apparatus comprising a vaeuum tube having at least a cathode, a grid, and an anode operative during the application of a voltage between anode xand mathode, .means ior suddenly interrupting wperation 'by x:utting oif -the anodecathode voltage, anti.meansfor applying a vo1tagesourceand a .current detecting means in series:betvveen the;grid and anode operative to indicate the:presence ofemission current from the grid du:cing -.theinterval -after the anode-cathode voltage:haybeen :cutoff andbefore the grid has cooled2appreciably. 1

4. In apparatus comprising a vacuum tube havingaeathode, a gridandan anode operative during =the .application of recurrent voltage impulses between anode and cathode, .a voltage sourcea unilateral impedance, and .current de- J:ecting means eonnected .in series between the grid and .the anode operative .to indicate the presence-0f .emission current.from the g1id during :the intervals betvv.eensaid voltage impulses.

ZIGMOND -W. 'WILCHINSKY.

N o references cited. 

